This invention relates generally to an improved device for use in depositing condensation coatings on various substrates. More particularly, the invention relates to an improved modular deposition chamber for depositing para-xylylene polymers on electrical component parts and the like
Para-xylylene polymers are employed as coatings for various electronic components due to their desirable physical and electrical properties. One advantage of poly-para-xylylene coatings is that e thin layers of such coatings are capable of exhibiting highly desirable physical and electrical properties Because para-xylylene coatings are applied in very thin layers, heat tends to dissapate rapidly from the underlying components. Thus, the coated components cool down quickly and are less prone to temperature related degredation than similar components bearing other types of coatings.
In further contrast to conventional polymer coatings, para-xylylenes are generally not prepolymerized prior to application on the coatable substrate. This is so because the para-xylylene polymers are not given to simple extrusion, melting or molding as are many of the conventional thermoplastics Additionally, because the para-xylylenes are generally insoluble in commonly used organic solvents, it is impractical to employ traditional solvent deposition techniques for applying poly-para-xylylene coatings.
Accordingly, in most commercial applications, para-xylylene polymers are deposited on desired substrates by a pyrolytic deposition process known specifically as the "parylene process." Such process begins with the vaporization of a cyclic di-para-xylylene dimer. The dimer is pyrolytically cleaved at temperatures of about 400 to 750.degree. C. to form a reactive para-xylylene monomer vapor. Thereafter, the reactive monomer vapor is transferred to a deposition chamber wherein the desired substrates are located. Within the deposition chamber, the reactive monomer vapor condenses upon the desired substrates to form a para-xylylene polymer or co-polymer film.
Any monomer vapor which fails to condense within the deposition chamber is subsequently removed by a cold trap which is maintained at approximately -70.degree. C.
The entire parylene process is generally carried out in a closed system under constant negative pressure. Such closed system may incorporate separate chambers for the (a) vaporization, (b) pyrolysis, and (c) deposition steps of the process, with such chambers being connected by way of appropriate plumbing or tubular connections.
A primary consideration in the parylene deposition process is the achievement of uniform coating thickness on the desired substrates. Unlike conventional polymer coating systems, the condensation deposition of parylene coatings is capable of depositing even ultrathin films without running or uneven areas resulting upon the substrates, provided that the monomer vapor is homogeneously and evenly distributed on the surface of the substrate. Thus, the design and functioning of the deposition chamber is critical to the achievment of uniform vapor distribution with resultant even coating deposition.
Another important consideration in the parylene deposition process is the minimization of waste. Currently, parylene raw materials may cost as much as $400 to $600 per pound. Thus, there exists substantial economic motivation to preserve and conserve the parylene materials during the coating process. One particular area in which a great deal of material is wasted is on the various internal structures of the prior art parylene deposition chambers It must be appreciated that the condensation deposition of coatings is not substrate selective -- i.e. the vapors have no way of seeking out only the desired substrate Instead, the monomer vapor will condense and polymerize on any reduced temperature object with which it comes in contact. As a result, the entire inner surface, of the chamber, and all of the objects positioned therein will become covered with the parylene coating Thus, the interior of the chamber and any existing hardware must be cleaned periodically to remove wasted parylene polymer
The parylene deposition chambers employed in the prior art have generally provided less than optimal coating uniformity due to inferior distribution and homogeneity of the vapor within the deposition chamber. Also, because of the particular chamber design, the prior art deposition chambers are associated with a great deal of waste of the parylene chemicals
At least one deposition chamber of the prior art employs a system of baffles, positioned adjacent a monomer vapor inlet line, so as to disperse the flow of vapor as it enters the deposition chamber. Such baffles are intended to uniformly distribute the monomer vapor throughout the interior of the deposition chamber thereby insuring uniform coating thickness on the desired substrates. In practical application, however, the various baffle designs employed in the prior art devices have failed to provide truly optimal vapor distribution within the chamber As a result, less than optimal coating uniformity has been realized Additionally, the presence of such baffles occupies otherwise useable space within the chamber and results in greater surface area within the chamber. Such increased surface area, accordingly, increases the amount of parylene waste due to the nonselective deposition of the polymer on the baffles as well as on the desired substrates
Also, the deposition chambers of the prior art incorporate substrate holding racks which are supported only by one or more members extending from the bottoms thereof. The absence of any support member fixing the top end of such holding rack to the surrounding deposition chamber structures results in a rather unstable arrangement. Specifically, when relatively heavy parts are unevenly distributed on the upper shelves of the holding rack such rack may tend to lean against the surrounding baffles or deposition chamber walls.